The invention relates to an arrangement for restituting a clock, included in the receiver of a system for the transmission of data by n-valent phase modulation of a carrier and used to sample the demodulated signals obtained by demodulation of the received signal with the modulation frequency by means of signals, which are in phase and in quadrature-phase with a local carrier, the arrangement for restituting a clock comprising a circuit for controlling a local clock on the zero-passages of a signal.
This arrangement has for its object to restitute in the receiver, from the received phase-modulated signal, a local clock which is in synchronism with the clock used in the transmitter for modulating the carrier, in order to sample, with a proper phase, the two orthogonal signals obtained by modulation of the signal received from a locally generated carrier. Such a sampling with a local clock which is in synchronism with the clock of the transmitter, is indispensable for the proper restitution of the phases and, consequently, of the transmitted data.
It is known to use one or the other of two orthogonal signals which are demodulated from the locally generated carrier, as the signal whose zero-passages are used to control the local clock. But this known method is only suitable for use when the phase of the generator generating the local carrier is accurately locked on the phase of the generator generating the carrier in the transmitter. If, for some reason or another, a phase control between the carrier of the transmitter and the receiver has not been provided or is not possible, a variable phase deviation is produced between the carriers of the transmitter and the receiver, resulting in that the instants at which the demodulated signals pass through zero are poorly defined and are not suitable for synchronizing the clock of the receiver with the clock of the transmitter. In certain types of receivers it is further not possible to obtain phase control between the carriers of the transmitter and the receiver before synchronization of the clocks has been obtained, which presents a problem at the start of a data transmission.